1. Field of the Invention
The present invention relates to an optical disc on which an information signal is recorded, and to a reproduction apparatus for the optical disc. More specifically, it relates to an optical disc which is used for recording video information which includes video data, audio data, and image data, and to a reproduction apparatus for the disc.
2. Description of the Related Art
Video applications, such as movies, are one kind of application which can be achieved using optical discs, such as laser discs, and a suitable reproduction device. These video applications are usually referred to as "video titles".
As the supply of video titles becomes increasingly internationalized, "multilingual video titles" are starting to appear on the market. Here, the expression "multilingual" refers to the user being able to select subtitles for one out of a plurality of languages which are then reproduced with the moving picture information. As one example, a multilingual video title aimed at the U.S. and European markets can include separate dubbing soundtracks and sub-picture information for English subtitles, German subtitles, French subtitles, and Spanish subtitles, so that users can watch a movie in their specified language.
One of the technologies which underpins the realization of "multilingual video titles" is the use of MPEG (Moving Pictures Experts Group) stream data as format for the data stored on the disc. MPEG streams include video information which has been compressed at a high compression rate according to MPEG methods, and can also include multiple sub-picture channels and multiple audio channels which are selectively reproduced. This is to say, "multilingual video titles" can be achieved by storing the moving picture information as the video information, dubbing soundtracks for each of the featured languages as the multiple channels of audio information, and subtitles for each of the featured languages as the multiple channels of sub-picture information.
Another of the technologies which underpins the realization of "multilingual video titles" is the use of a process called OSD (On-Screen Display) processing performed by the disc reproduction apparatus. Here, OSD processing refers to the decoding of the image data included as sub-picture information and the mixing of the image data with the moving picture on the display. This is to say, when the user selects the language he/she would like reproduced, the disc reproduction apparatus selects and reproduces the sub-picture information on one of the channels in accordance with the selected language, with the sub-picture information being reproduced together with the moving picture. The image data used here for displaying subtitles is known as "tel-op" data.
The position at which the sub-picture information is superimposed onto the moving pictures is decided by the title producer when developing the title. As a result, during reproduction the sub-picture information can be positioned at the producer's desired position. Here, should the producer indicate a display command for a coordinate position around the chest of one of the characters in the main image, the disc reproduction apparatus may respond to that command by displaying subtitles showing the character's name which are superimposed over the chest area of the character. Here, when a plurality of characters are concurrently displayed, the subtitles for characters' names can be displayed at positions corresponding to the respective characters, making such subtitles extremely effective.
Another recent trend in video titles, apart from the development of "multilingual titles", is the development of "wide-screen TV-adapted titles". Adaptation for wide-screen television refers to the development of titles which have a high visual impact when displayed on a wide-screen television with an aspect ratio of 16:9, with the objective of giving commercially available video titles the same big-screen feeling attained from watching movies at a movie theater.
One of the technologies underpinning the adaptation of titles for wide-screen TVs is, of course, the development of wide-screen TV sets. Wide-screen TV sets have a screen whose aspect ratio is 16:9 and perform interpolation for a NTSC (National Television System Committee) signal whose image content has an aspect ratio of 4:3 to display an image whose aspect ratio is 16:9. This is to say, video information which is stored on a disc is processed so as to squeeze its image content in the horizontal direction to enable the information to be optimally displayed at a 16:9 aspect ratio. As described above, wide-screen TV sets perform interpolation for this video data which has been squeezed in the horizontal direction and by doing so display the video information with an aspect ratio of 16:9.
Another of the technologies underpinning the adaptation of titles for wide-screen TVs is display mode transformation processing by disc reproduction apparatuses. Here, display mode transformation processing refers to image interpolation which enables a video title adapted to a 16:9 aspect ratio to be reproduced by a conventional television set which has an aspect ratio of 4:3. As described above, video titles which are adapted to wide-screen TV use are processed so as to give an optimal display when displayed at an aspect ratio of 16:9. To do so, the video information is shrunk in the horizontal direction. As a result, conventional TV sets which cannot perform horizontal interpolation during display end up displaying the image shrunk in the horizontal direction as it is. In order to perform compensation, display mode transformation processing is executed by disc reproduction apparatuses, and by switching the display format (display mode) of the image signal for the main image, the image content can be arranged within the limited display area of a conventional TV set.
Here, "pan scan display mode" and "letterbox display mode" are representative examples of display modes. In the former, pan scan display mode, video which is adapted to wide-screen television is slid by given distances in the vertical and horizontal directions in accordance with the FCVO (Frame Center Vertical Offset) and the FCHO (Frame Center Horizontal Offset) which are standardized under MPEG, before the left and right edges of the image are trimmed to give an image which will fit onto a TV screen with an aspect ratio of 4:3. Putting this into other words, pan scan display mode involves a compensatory cutting of the image content of each frame so that the image can be displayed on a TV screen with an aspect ratio of 4:3. In the latter, letterbox display mode, the image recorded with an aspect ratio of 16:9 is reduced and displayed in the central area of a 4:3 aspect ratio TV screen, with a corrective image being added above and below the central belt-like area where the image is displayed. Putting this in other words, letterbox display mode involves a compensatory reduction of the image content of each frame with an aspect ratio of 16:9 so that the image can be displayed on a TV screen with an aspect ratio of 4:3.
When pan scan display mode and letterbox display mode are compared, letterbox display mode has the distinct merit of being an easier mode to achieve. This is because in pan scan display mode, the producer must set the FCVO and FCHO values very carefully for each frame of video, so that unintentional and undesired losses of image data, such as the trimming of half a character's face, can be avoided.
Using the letterbox display mode and the pan scan display mode described above, video information which has been adapted for wide-screen TV can be favorably arranged onto a conventional TV screen with an aspect ratio of 4:3. However, there remains the problem for TV sets, disc reproduction apparatuses, and disc that when "multilingual video titles" which have been developed for wide-screen TV are displayed using the above modes, the superimposing position of the subtitles cannot always be performed in accordance with the producer's intentions.
When a disc reproduction apparatus has a video title adapted for wide-screen TV displayed on a conventional TV with an aspect ratio of 4:3, conversion to pan scan display mode is performed as described above, with this changing the position at which the video content is arranged. As a result, the sub-picture will end up being displaced from the desired position, which leads to a breakdown in the positional correspondence between items on the screen and the subtitles related to the items.
The following is a detailed explanation of the breakdown in the positional relationship between a sub-picture and actual video footage which will refer to a specific example of video footage. Here, when editing a video title which is a nature program, the producer wishes to superimpose subtitles onto a frame introducing bird life. Here, this one frame of the wide-screen TV-adapted image is as shown by the reference numeral y101 in FIG. 1A, with reference number y102 showing the subtitles which are to be superimposed onto the frame. These subtitles are to be superimposed at the feet of the bird in the image, with the producer indicating that subtitles indicating the word "Penguin" are to be superimposed at certain coordinates in the wide-screen TV-adapted image.
The combined image y103 shows the results of the superimposing of the subtitles y102 on the image y101. Should the producer only see image, he/she should be satisfied with the results of the superimposing, since in FIG. 1A the subtitles have been superimposed at the producer's desired position. Here, the results of the same superimposing for a main image displayed using pan scan mode are shown in FIG. 1B. From FIG. 1B, it can be clearly seen that the subtitles have been displaced from the intended position, since the superimposing has been performed with the "Penguin" subtitles at a position spatially defined with respect to the wide-screen TV-adapted image, despite the fact that the image has been trimmed to fit the standard TV screen. This is because the coordinates for superimposing the sub-picture on the main video image in pan scan display mode are different to the coordinates for superimposing the sub-picture on the main video image in wide-screen TV-adapted mode. Here, the use of coordinates for an optimal superimposing position in wide-screen TV-adapted mode when displaying according to pan scan display mode can often result in the displacement of the display of the sub-picture.
FIG. 1C shows the combined image which results from the disc reproduction apparatus switching to letterbox display mode. As can be seen, the "Penguin" subtitles once again end up displaced from the intended image content. This leftward displacement of the superimposing position is caused, as was the case with the displacement in pan scan display mode, by the use of different coordinate systems. Here, the use of coordinates for an optimal superimposing position in wide-screen TV-adapted mode when displaying according to pan scan display mode can often result in the displacement of the display of the sub-picture.
As shown in FIG. 1C, the subtitles end up being superimposed onto the border between the main image and the corrective image, with the top half of the subtitles being displayed on the main image and the bottom half being displayed on the corrective image. As shown by the enlargement indicated by the arrow y104, since the subtitles are displayed in black, the favorable color contrast of the top half of the subtitles is not achieved in the lower half.
Since the above displacement is due to the use of different display modes, it is possible to consider that the superimposing of the sub-picture on the main image would be better performed at a earlier stage of the process, which is to say before the conversion to pan scan or letterbox display. An example of when the main image and the sub-picture are combined before conversion into pan scan display mode is shown in FIG. 2A, with FIG. 2B showing the case when the main image and the sub-picture are combined after conversion into letterbox display mode. With the case shown in FIG. 2A, trimming the image for pan scan display mode results in the loss of the final two letters of "Penguin", while in FIG. 2B, reduction of the image for letterbox display mode results in the reduction of the subtitles, so that the "e" and "g" of "Penguin" and end up appearing as if they have been colored in, making the letters very difficult to read.
While combining the main image and sub-picture at an early stage in this way helps to overcome the problems of displacement, this improvement is surpassed by the loss of subtitles or by the difficulty of reading reduced characters, so that such methods cannot be considered practicable.